public void Filter_for_period()
{
var formHelper = new FormHelper();
PrepareHelper(formHelper);
context.CurrentControllerContext.Helpers.Add(formHelper);
context.Services.ViewEngineManager = ForTest.GetViewManager();
filter.Period = new DatePeriod();
var result = helper.FilterFor("filter.Period");
Assert.That(result, Is.StringContaining("calendar"));
}